The entire globe is moving fast to stop climate change. The Paris climate agreement enters into legal force on Friday, and then shortly afterwards comes a first global meeting to start implementing it in Marrakech, Morocco. (Yes, there’s also a U.S. election in there somewhere that could, er, complicate things.)

But this flurry of activity nonetheless faces a grim mathematical reality, a new report from the United Nations Environment Programme finds. In essence, while the Paris agreement sets extremely ambitious temperature goals — holding the world’s warming to “well below” 2 degrees Celsius above pre-industrial levels, and striving for a 1.5 degree limit — current policies and promises alone have little chance of attaining them.

Moreover, because of the unforgiving nature of carbon math — once you emit a ton of carbon dioxide, you can’t get it back, and it accumulates steadily in the atmosphere — there is exceedingly little time to change course and increase ambitions.

“It’s just too little, and it’s not happening quickly enough,” said Jacqueline McGlade, UNEP’s chief scientist. “If we don’t see emissions peaking by 2020, then the chances of getting to 1.5 degrees is vanishingly small.”

Such is the upshot of UNEP’s latest installment of its “emissions gap” report, perhaps the definitive study of how much the world is currently emitting, and how much it can emit to remain on course to meet its goals. The difference between the two comprises the gap.

Here are the details (warning, this stuff gets complicated quickly).

Right now, due to causes ranging from deforestation to transportation, the world is emitting about 52.7 billion tons, or gigatons, of carbon dioxide equivalents per year as of 2014. That’s mostly just plain carbon dioxide, but it also includes emissions of methane, nitrous oxide, and other greenhouse gases that are converted into units comparable to carbon dioxide. If you leave those out, the pure carbon dioxide emissions are about 36 billion tons per year.

However, to hold global warming below 2 degrees (at least with good odds), the world can emit no more than 1,000 gigatons of carbon dioxide from the year 2011 onwards — the famous carbon budget. And given that it’s 2016 already, that number has already shrunken a good bit, by about 150 gigatons. And of course, the carbon budget is even narrower to hold warming to 1.5 degrees Celsius.

This is the logic behind the inescapable emissions “gap”: If we want to hold global warming to 1.5 C, we need to be emitting only 38.8 gigatons of carbon dioxide equivalents by the year 2030. For 2 degrees C, there’s only slightly more leeway — 41.8 gigatons.

The promises countries have made under the Paris agreement don’t remotely get there — at best, they’d have us at about 53.4 billion tons in 2030. The emissions gap is therefore between 12 and 14 gigatons per year if you want to keep the planet at 2 degrees, and between 15 and 17 gigatons per year for 1.5 degrees, says UNEP.

“When you think that one gigaton is the equivalent of taking all European vehicles off the road for one year, and the gap is between 12 and 14 gigatons, you see what the scale of the problem is,” explains McGlade.

Thus, we’re way off course with very little time to turn things around. The world’s current promises, says UNEP, would allow the planet to warm by about 3 degrees C above pre-industrial levels.

And by the way: even these numbers for keeping warming below 1.5 or 2 degrees tend to assume something that many scientists think is dubious. They tend to rely on the assumption that we’ll bust through our carbon budgets but somehow get a second chance later in the century, once we create technologies that can somehow withdraw carbon dioxide out of the air again. These scenarios often have the world removing net amounts carbon dioxide from the atmosphere after 2050, rather than putting more there. It’s far from clear that will actually happen, at least at the scale that would be required.

Granted, McGlade underscores that this doesn’t mean there is no hope — it just means that the world has to do massively more, and it has to do it quickly. Every year, we bank more carbon emissions in the atmosphere. Every year we fail to bend the curve sharply downwards, it becomes that much harder to get on the right course.

This is why, McGlade asserts, emissions have to peak and start to decline by 2020 — just four short years from now — or else the 1.5 degree target could be gone.

The Marrakech meeting beginning next week won’t be the place to raise global ambitions under the Paris process — at the earliest, it appears that may happen in 2018. But McGlade says in the meantime, there are other actions that can shrink the emissions gap, including moves by sub-national players like major corporations, cities, and large scale land users in the agricultural and forestry sectors. Precious gigatons can be won back in this way, potentially narrowing the gap.

“Cities don’t have to wait, companies don’t have to wait, society doesn’t have to wait,” says McGlade.